The term “HVAC system” will be used herein to refer to any system capable of heating, cooling, and/or ventilating an indoor space. The heating, cooling, and/or ventilation parameters of an HVAC system can typically be adjusted by a thermostat, which might also be referred to as an indoor comfort control. The thermostat is typically connected to an HVAC system controller that controls the HVAC system based on input received from the thermostat. At least a portion of the functions carried out by the thermostat might be performed by a programmable microprocessor or a similar component. The microprocessor might be connected to a temperature sensor that can sense the temperature of the space in which the thermostat is located and send a suitable signal to the microprocessor indicating the temperature in that space. The microprocessor might receive power from the HVAC system controller and might connect to a suitable battery power source as a backup. The thermostat may be operated to change the temperature set point, cause the system to operate in a heating mode or a cooling mode, operate only a fan of the HVAC system, and/or perform other functions. In some cases, the term “HVAC system” might refer to the thermostat and the HVAC system controller in combination with the air heating and cooling components, and in other cases, the term “HVAC system” might refer to the air heating and cooling components independently of the thermostat and the HVAC system controller.